A metal oxide silicon (MOS) transistor has a structure in which a gate is formed on a gate dielectric layer which, in turn, is deposited on a semiconductor substrate. The MOS transistor is a unipolar transistor in which current flows using electrons or holes. A positive or negative voltage can be applied at the gate of the MOS transistor. An inverse bias is not required. The input impedance of the gate is very high. Also, the MOS transistor can be manufactured through simple and cost effective fabrication techniques and can be highly integrated. Furthermore the MOS transistor exhibits low power consumption. Various methods for fabricating a MOS transistor have been described in the U.S. Pat. No. 6,458,639, U.S. Pat. No. 6,297,535, and U.S. Pat. No. 5,648,284.
FIG. 1 illustrates a conventional MOS transistor. Referring to FIG. 1, a gate oxide layer 12 and a gate 13 are formed on a semiconductor substrate 11. Source and drain regions 14a and 14b are formed within the semiconductor substrate 11 at opposite sides of the gate 13.
Typically, this MOS transistor is manufactured by forming the gate oxide layer 12 on the semiconductor substrate 11 at a uniform thickness; depositing a polysilicon layer as a gate material layer, patterning the polysilicon layer through photolithography, and performing etch processes to form the gate 13. Impurities are ion-implanted into the substrate 11 to form the source and drain regions 14a and 14b within the substrate 11 at opposite sides of the gate 13.
However, since the thickness of the gate oxide layer 12 is uniformly formed, when applying voltage to the drain region 14b after a channel is formed below the gate 13, the depletion layer 100 becomes thicker around the drain 14b than around the source region 14a due to the potential difference. Accordingly, if the electrons (e) emitted from the source region 14a flow to the drain region 14b, the speed of the electrons (e) becomes faster when the electrons reach the depletion layer around the drain region 14b such that a pinch-off point (A) occurs. The electrons (e) penetrate the gate oxide layer 12 around the pinch-off point (A), according to the speed of the electrons (e), the gate voltage, and so on. These electrons (e) become thermal electrons emitting heat due to frequent collisions with the interface of the gate oxide layer 12 and the silicon and their fast speed. The emitted heat may damage the gate oxide layer 12, resulting in degradation of the properties and/or the reliability of the transistor.